Electronic lens for a photoelectron multiplier

ABSTRACT

An improvement in a crossfield photoelectron multiplier that has a plurality of electron emitting dynodes parallel and longitudinally extended with respect to the anode. The improvement anode has a series of parallel fins forming channels and controlling the electron field between the anode and the dynodes and between the anode and the cathode.

mte States Patet 1151 3,641,352 Fisher Feb. 8, 1972 [54] ELECTRONIC LENS FOR A [56] References Cited PHOTOELECTRON MULTIPLIER UNITED STATES PATENTS [72] Inventor: Mahlon B. Fisher, Skaneateles, NY. 2,163,700 6/1939 Ploke et al. ..313/95 2,664,515 12/1953 ,Smith ...313/105 [731 Assgneei The i 2,762,920 9/1956 Wiley... ...313/105 gv by the SKI/em! 3,431,420 3/1969 Fisher ..250 2o7 orce 22 d; 1 19 9 Primary Examinerwalter Stolwein 1 Attomey-Harry A. Herbert, Jr. and Julian I.v Siegel [21] Appl. No.: 885,577 r [57] ABSTRACT [52] 11.8. CI .....250/207, 313/95, 250/213 VT An improvement in a crossfleld photoelectron multiplier that 51 1m. (:1. ..H0lj 39/12, HOlj 39/50 has a plurality of electron emitting dynodes Parallel and [58] Field of Search ..250/207, 213 VT, 21 1; 313/95, gimdinally extended with respect amdeimPmve- 313/105 ment anode has a series of parallel fins forming channels and controlling the electron field between the anode and the dynodes and between the anode and the cathode.

2 Claims, 5 Drawing Figures M We ELECTRONIC LENS FOR A PHOTOELECTRON MULTIPLIER BACKGROUND OF THE INVENTION periodic electrostatic lens system with the effective lens region located very near to the dynode surface causing the object plane to be substantially less than a focal length away from the lens region. A real image is, therefore, not possible with such a lens system. However, the use of a finned electrode at the anode plane results in a periodic transverse electric field near the midpoint of the electron trajectory. This provides an optimum location for the electrostatic lens region since both a real image and unity magnification re possible.

There are three significant differences between the finned anode and finned dynode focusing structures. Each fin on the anode defines the center of a channel whereas the fins on the dynode define the edge of a channel. The location of the electrostatic lens region in the finned anode structure is at the approximate midpoint of the electron trajectory for a given pair of dynodes while the lens region in the case of the finned dynode geometry is very near the beginning of the trajectory. Finally, only the fringing field external to the finned anode structure is effective in establishing the electrostatic lens while the entire transverse electric field (both inside the fins and external to the fins) contributes to the electrostatic lens in the case of the finned dynode structure.

SUMMARY OF THE INVENTION Symmetry and improved resolution in a photoelectron multiplier is obtained by constructing an anode that includes a series of extensions or fins in the longitudinal direction of the anode which opposes a series of electron emitting dynodes. The fins form channels for the electric field and define the centers of the channels.

It is therefore an object of the invention to provide an improved photoelectron multiplier that offers the formation of a real image.

It is another object of the invention to provide a photoelectron multiplier that provides unity magnification.

It is still another object to provide a photoelectron multiplier that has improved resolution for use in an optically mosaic receiver.

These and other advantages, features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiment in the accompanying drawings, wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a photoelectron multiplier forming an embodiment of the invention.

FIG. 2 shows the anode structure taken at 2-2 of FIG. 1 together with a diagram of the electric field.

FIG. 3 is a graph illustrating the transverse electric field distribution as a function of the height above the dynode.

FIG. 4 is a diagram showing an electron trajectory in a fin anode structure with on axis emission.

FIG. 5 is a diagram showing an electron trajectory in a tin anode structure with off axis emission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, there is shown a photoelectron multiplier which can embody the invention. Incident light 11 passes through metal screen or grating 13 and strikes photosensitive cathode 15. The purpose of grating 13 is to preserve electrical contact with the adjoining electric parts (to be explained later), and still pass the incident light on to cathode 15 in order to create secondary emission of electrons. The electrons move to adjacent dynode l7 and then to remaining dynodes 18-20 in the path shown by the arrows and finally to output circuit 23. The potential difference between cathode l5 and anode 43 and between dynodes 17-20 and anode 14 is maintained by voltage source 25 together with variable resistors 27-34. The direction of the resulting electric fields is shown by arrow 37. By adjustment of the variable resistors the potential increases stepwise as the electrons advance through the series of dynodes. Further adjustment is made by positioning the dynodes increasingly closer to the anode. A magnetic field perpendicular to the electric field and. to the longitudinal axis of the photoelectron multiplier is shown as the feather of arrow 39.

Referring to FIG. 2, there is shown the anode geometry where fins 41 are connected to the main body of anode 43 forming channels 44. Between fins 41 and dynode 45 an electric field is established as shown by field lines 47 the fins forming the center of the channels with respect to the electric field.

An example of the finned anode geometry is considered in which the parameters are:

Channel Width d=0.200 inches Fin Height b=0.050 inches Anode-Dynode Spacing 0 =0.600 inches Magnetic Field B,= I60 gausa Anode Voltage V,=5,000 volts Cyclotron Angular Frequency m,=2.81 l0' Transverse electric field is such that the polarity of the field must be reversed and that the centerline of a channel corresponds to the location of an anode fin. The peak of the electron trajectory or the maximum y value is given by:

ZnE

ymax 0 In the region near the finned anode, B, may be approximated by the first term of the appropriate expansion for the sine and sine functions 2: If LIZ! E (x, y)=A sin sinh where A and y, are to be determined from FIG. 4.

The equations of motion in the x and y direction must be used to express the electric field as a function of time for the evaluation of the momentum integral. In the case of the equation of motion in the x direction, it is assumed that no motion within the lens region which is due to the forces of the focusingfield (thin lens approximation). That is, x is independent of time within the lens region and the transverse position at which an electron enters the lens region is given by x,,-x +v t where is the transverse position at which the electron enters the focusing region. .r is the transverse position at which the electron left the dynode surface, and is the initial velocity component in the direction.

The appropriate expansion near y=a( m,t-1r) is y= t t-)l.

The momentum integral for the case of the finned anode structureis m .2. 11:22 .u. (my) (AI d [1 2 Ad:

where y is the maximum height of 'tii' ioian trajectory. This expression may be simplified somewhat as follows:

l mu

The integration may now be performed as follows:

The upper limit, 1 is the time required to reach y and is simply rrlm The effective focusing field exists from y 4b to the maximum height of the electron trajectory which is y=l.4b. That is, the electron enters the focusing field at a height of 4b, rises through the field and leaves the focusing field again at a height of 4b. The position and velocity of an electron at the entrance to the focusing region must now be calculated and then the change in momentum in passing through the focusing field may be calculated. Finally the position at which the electron strikes the next dynode may be calculated. This is done for three initial velocities for the case of electrons which have a point on the dynode halfway between the center and edge of a channel.

The magnitude of the transverse electric field at the y'=y,,.,, (the constant A) is obtained from FIG. 3 as 650 volts/inch. The time, T at which the electron enters the focusing field (the lower limit of the momentum integral) is 0.9Xlsecond.

The upper limit, t,, is l.l2 l0 seconds. Three trajectories were calculated for the condition that y,,, is zero, +0.708Xl0 m./sec. and -707 l0 m./sec. These values correspond to electrons emitted normal and :45 to the dynode surface with 3 ev. initial energy. The resulting trajectories are shown in projected form in FIG. 4 for the case of an initial position in the center of a channel.

It may be observed that a diverging cone of electrons emitted from a point on the center of a channel are brought to focus slightly in front of the next dynode in this example. A second calculation for a diverging cone of electrons emitted from a point halfway between the center and the edge of a channel was performed and the results are shown in FIG. 5. In FIGS. 4 and 5, the lengths and position of anode fins 41 represent the effective length of the fins and the efi'ective position of the electrostatic lens. In the case of FIG. 5, the electron beam is essentially collimated and remains in the correct channel. That is, the image plane is at infinity since the ob ect plane (dynode surface) is located approximately at a focal point of the lens system.

I claim:

1. In an electron photomultiplier having:

a. a photosensitive cathode aligned to receive incident light;

b. A plurality of longitudinally aligned and longitudinally spaced dynodes;

c. an anode longitudinally coextensive with the cathode and dynodes;

d. means for producing a variable electric field in the space between the anode and the cathode and between the anode and the dynodes;

e. means for producing a magnetic field perpendicular to the electric field and in the same space as the electric field;

the improvement comprising a plurality of longitudinally aligned extensions of the anode the plurality of extensions being parallel to each other and perpendicular to the anode forming a plurality of electrostatic channels in which the extensions are at the center thereof.

2. In an electron photomultiplier having:

a. a photosensitive cathode aligned to receive incident light;

b. a plurality of longitudinally aligned and longitudinally spaced dynodes;

c. an' anode longitudinally coextensive with the cathode and dynodes;

means for producing a variable electric field between the anode and the cathode and between the anode and the dynodes;

e. means for producing a magnetic field perpendicular to the electric field;

the improvement comprising:

a. a plurality of longitudinally aligned extensions of the anode the plurality of extensions being parallel to each other and perpendicular to the anode; and

b. a series of segmented channels defined by a plurality of longitudinal extensions of each dynode.

* I Q i 

1. In an electron photomultiplier having: a. a photosensitive cathode aligned to receive incident light; b. a plurality of longitudinally aligned and longitudinally spaced dynodes; c. an anode longitudinally coextensive with the cathode and dynodes; d. means for producing a variable electric field in the space between the anode and the cathode and between the anode and the dYnodes; e. means for producing a magnetic field perpendicular to the electric field and in the same space as the electric field; the improvement comprising a plurality of longitudinally aligned extensions of the anode the plurality of extensions being parallel to each other and perpendicular to the anode forming a plurality of electrostatic channels in which the extensions are at the center thereof.
 2. In an electron photomultiplier having: a. a photosensitive cathode aligned to receive incident light; b. a plurality of longitudinally aligned and longitudinally spaced dynodes; c. an anode longitudinally coextensive with the cathode and dynodes; d. means for producing a variable electric field between the anode and the cathode and between the anode and the dynodes; e. means for producing a magnetic field perpendicular to the electric field; the improvement comprising: a. a plurality of longitudinally aligned extensions of the anode the plurality of extensions being parallel to each other and perpendicular to the anode; and b. a series of segmented channels defined by a plurality of longitudinal extensions of each dynode. 